Dyeing of cellulose-containing textile materials by means of vat dyes is generally known. In order to lend vat dyes, which are insoluble in water, the necessary substantivity, i.e. to fix it on the textile material, it is first necessary to change it into a substantive water-soluble leuco form by reduction (vatting) and then to develop it again into a dye pigment by oxidation.
In their leuco form, most vat dyes are distinguished by a high affinity to the fiber and therefore by high bath exhaustion of approximately 70 to 95%.
In contrast thereto, leuco-indigo is only absorbed by the fiber by 10 to 20% in a single dyeing process. Dyeing with indigo in accordance with a discontinuous extraction process (dyeing from the bath) is problematical because of this low bath exhaustion.
It is therefore customary to perform dyeing with indigo continuously "in several operations". With this method which only permits dyeing of yarn in the form of a cable or a warp sheet, but not the dyeing of piece goods or wound bodies, the vatted indigo from several (mostly five or six) dye liquors of large volume and low dyestuff concentration is applied by repeated, short (approximately 10 to 20 sec) dipping as well as squeezing and oxidizing in interspersed air passages. However, dyeing in several operations is only considered to be a stopgap measure, since it requires dyeing machinery of inefficient size, and smaller dye batches can hardly be realized. In addition, a considerable penetration of textile material is the result of the repetition of the dye process, so that extremely ring-dyed material (in particular in dark colors), which is used in the manufacture of jeans and is a prerequisite for generating the "stone-washed" effects in jeans washing, is only insufficiently obtained.
A further problem in dyeing with indigo is that the dyes obtained only have insufficient crocking fastness because of the low bath exhaustion, since dyestuff remaining in the dye bath is deposited on the textile material after oxidation and is not fixed.